DOCBOOK: DocBook Extension syntax

I'm not totally sure if this question should be asked here or on the
docbook-apps list, but it seems more related to the DTD, so I'll try
here.

I'm trying to write an extension to DocBook that adds several attributes
to programlisting and extends its content model slightly.  So far, I
have been unable to generate a DTD that parses correctly; what follows is
my current attempt:

<!--
  DBLP.DTD; a literate programming DTD based on DocBook
  PUBLIC "-//Mark Wroth//DTD DocBook V4.1 Literate
Programming Extension 1.0//EN"
-->
<!ENTITY % programlisting.element "IGNORE">
<!ENTITY % local.programlisting.attrib "
 file         
CDATA  #IMPLIED -- file name for output file --
 continuedfrom IDREF  #IMPLIED
 continuedin   IDREF  #IMPLIED "> 
<!ENTITY % docbook PUBLIC "-//OASIS//DTD DocBook
V4.1//EN">
%docbook;

<!ELEMENT ProgramListing - - 
  ((CO | LineAnnotation | XRef | %para.char.mix; )+) >

The basic intention is to add xref to the content model, and the
three attributes file, continuedin, and continuedfrom.  Looking at
dbpool.mod, I would have predicted that this dtd would cause the original
definition of programlisting to be ignored, substituting the new one,
while adding the attributes.  However, neither onsgmls nor openjade
believe this is a syntactically correct DTD.

What am I doing wrong?

Mark B. Wroth

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